(1) Field of the Invention
The present invention relates to a solid state imaging device capable of parallel reading of data from a plurality of pixel cells, and especially to a technique for suppressing occurrences of crosstalk.
(2) Description of the Related Art
Solid state imaging devices have been commonly used as image capturing devices such as digital still cameras. One type of solid state imaging device is a MOS-type solid state imaging device having a sensor unit composed of a plurality of pixel cells arranged two-dimensionally (in an array, for example). In order to provide digital still cameras with additional values, such as continuous shooting capability and video shooting capability, it is desirable to improve MOS-type solid state imaging devices in the data readout rate (i.e., the speed at which data is read) from the sensor unit as much as possible.
Various suggestions have been made to improve MOS-type solid state imaging devices in the data readout rate (see for example, JP Patent application publication No. 2006-295620). The following describes the MOS-type solid state imaging device disclosed in JP Patent application publication No. 2006-295620, with reference to FIG. 1. FIG. 1 shows, out of an array of a plurality of pixel cells, two pixel cells 901a and 901b disposed on the same column.
As shown in FIG. 1, each of the two pixel cells 901a and 901b similarly has the following structure. That is, the pixel cell 901a (901b) includes a photodiode 908a (908b) and three transistors 910a, 911a, and 912a (910b, 911b, and 912b). The photodiode 908a (908b) is connected at one end to the source of the transfer transistor 910a (910b). The drain of the transfer transistor 910a (910b) is connected to the gate of the amplification transistor 911a (911b) and also to the source of the reset transistor 912a (912b). The pixel cell 901a (901b) also includes a charge storage portion 909a (909b) disposed between the drain of the transfer transistor 910a (910b) and the gate of the amplification transistor 911a (911b). The gate of the transfer transistor 910a (910b) is connected to a corresponding one of transfer control signal lines Trans all extending in the direction of the X axis shown in the figure (hereinafter, simply “X direction”), whereas the gate of the reset transistor 912a (912b) is connected to a corresponding one of reset signal lines RS all extending in the X direction.
In addition, the drains of the amplification transistors 911a and 911b and of the reset transistors 912a and 912b are commonly connected to a corresponding one of pixel selecting lines 913 all extending in the Y direction. The source of the amplification transistor 911a included in the pixel cell 901a is connected to a corresponding one of output signal lines 904a all extending in the Y direction.
On the other hand, the source of the amplification transistor 911b included in the pixel cell 901b is connected to a corresponding one of output signal lines 904b all extending in parallel to the output signal line 904a. 
With reference to FIG. 2, the following now describes a method for driving a conventional MOS-type solid state imaging device having the above structure.
As shown in FIG. 2, in the initial stage of the driving, the pixel selecting line 913 is in the OFF state, the charge storage portions 909a and 909b in the pixel cells 901a and 901b are at the LOW level, and the amplification transistors 911a and 911b are in the OFF state (the state of the amplification transistors 911a and 911b are not shown in the figure).
Next, the pixel selecting line 913 is switched to the ON state and the reset signal lines RS of the rows targeted for the subsequent data reading are switched to the ON state. As a result of this operation, the charge storage portions 909a and 909b are reset to HIGH.
Next, the transfer control signal lines Trans are switched to the ON state, so that electrons resulting from photoelectric conversion by the photodiodes 908a and 908b are separately transferred to the charge storage portions 909a and 909b, which causes the respective charge storage portions 909a and 909b to change in the potential level. The respective amounts of change in the potential level are amplified by the amplification transistors 911a and 911b and then output to the output signal lines 904a and 904b, respectively.
In the above manner, two rows on the same column are selected concurrently and data is read from the two pixel cells 901a and 901b concurrently. With the above-described structure and driving method, the MOS-type solid state imaging device disclosed by the JP patent application publication No. 2006-295620 achieves to increase the data readout rate.
However, the conventional MOS-type solid state imaging device described above involves the following problem. That is, at the time of concurrent reading of data from the two pixel cells 901a and 901b, undesirable coupling occurs between the charge storage portion 909a of the pixel cell 901a and the output signal line 904b as well as between the charge storage portion 909b of the pixel cell 901b and the output signal line 904a, which causes crosstalk. A specific description is given below with reference to FIGS. 1 and 2.
As shown in FIG. 2, in response to that the transfer control signal line Trans is switched to the ON state, electric charge is transferred, so that the potential level of the charge storage portion 909a changes, which in turn changes the potential level of the output signal line 904a. Ideally, the potential level of the output signal line 904b connected to the pixel cell 901b remains unchanged unless the photodiode 908b of the pixel cell 901b has received incident light and thus electric charge has been stored.
As shown in FIG. 1, however, the conventional MOS-type solid state imaging device has the output signal line 904a disposed above or in the vicinity of the pixel cell 901b. This positional relation causes a parasitic capacitance 914 between the output signal line 904a and the charge storage portion 909b. Via the parasitic capacitance 914, the change in the potential level of the output signal line 904a is propagated to the charge storage portion 909b and eventually output to the output signal line 904b. 
As a result, a signal responsive to the amount of light received by the photodiode 908a of the pixel cell 901a is output to the output signal line 904b of the neighboring pixel cell 901b, even if no light is received by the photodiode 908b of the pixel cell 901b. As described above, the conventional MOS-type solid state imaging device involves the risk of crosstalk caused by the above-described mechanism.